(1) The work done in changing a physical system from one distribution of masses and velocities (state I) to another (state II), if the same amount of work can be recovered by the system's changing from state II to state I. Such a system is called a reversible mechanical system. (2) The work done in changing a physical system from one distribution of masses and velocities (state I) to another (state II), if the work done is a function only of the initial and final configurations and not of the trajectories along which the work was done. In speaking of systems composed of point masses, the phrase a function only of the initial and final coordinates of the masses is often used. The two definitions are almost equivalent, and both are referred to as definitions of mechanical potential energy or, if no confusion is likely, as definitions simply of potential energy. (3) The difference in mechanical potential energy of a body at two points P and Q is the amount of work done in moving that body from P to Q. It is, for a point mass, the product of the mass of the body by the negative of the difference in potential at the two points.